Staged combustion cycle
The staged combustion cycle, also called topping cycle or pre-burner cycle, is a thermodynamic cycle of bipropellant rocket engines. Some of the propellant is burned in a pre-burner, and the resulting hot gas is used to power the engine's turbines and pumps. The exhausted gas is then injected into the main combustion chamber, along with the rest of the propellant, and combustion is completed.
The advantage of the staged, or "closed", combustion cycle is that all of the engine cycles' gases and heat go through the combustion chamber. An alternative design, called a gas-generator cycle, exhausts the turbopump driving gases separately from the main combustion chamber, which leads to a few percent of loss of efficiency in thrust.
Another advantage that staged combustion gives is an abundance of power which permits very high chamber pressures that allow high expansion ratio nozzles. These nozzles give better efficiencies at low altitude.
The disadvantages of this cycle include harsh turbine conditions, exotic plumbing to carry the hot gases, and complicated feedback and control. In particular, running the full oxidizer stream through both a pre-combustor and main-combustor chamber (oxidizer-rich staged combustion) produces extremely corrosive gases. Therefore, most staged-combustion engines are fuel-rich, as in the schematic.
Staged combustion (Замкнутая схема) was first proposed by Alexey Isaev in 1949. The first staged combustion engine was the S1.5400 (11D33) used in the Soviet planetary rocket, designed by Melnikov, a former assistant to Isaev. About the same time (1959), Nikolai Kuznetsov began work on the closed cycle engine NK-9 for Korolev's orbital ICBM, GR-1. Kuznetsov later evolved that design into the NK-15 and NK-33 engines for the unsuccessful Lunar N1 rocket. The non-cryogenic N2O4/UDMH engine RD-253 using staged combustion were developed by Valentin Glushko around 1963 for the Proton rocket.
After the failure of the N-1, Kuznetsov had been ordered to destroy the NK-33 technology, but instead he warehoused dozens of the engines. In the 1990s, Aerojet was contacted and eventually visited Kuznetsov's plant. Upon meeting initial skepticism about the high specific impulse and other specifications, Kuznetsov shipped an engine to the US for testing. Oxidizer-rich staged combustion had been considered by American engineers, but deemed impossible. The Russian RD-180 engine, purchased by Lockheed Martin (subsequently by United Launch Alliance) for the Atlas III and V rockets, also employs this technique.
Hydrogen peroxide/kerosene fuelled engines such as the British Gamma of the 1950s may use a closed-cycle process (arguably not staged combustion, but that's mostly a question of semantics) by catalytically decomposing the peroxide to drive turbines before combustion with the kerosene in the combustion chamber proper. This gives the efficiency advantages of staged combustion, whilst avoiding the major engineering problems.
The Space Shuttle Main Engine is another example of a staged combustion engine, and the first to use liquid oxygen and liquid hydrogen. Its counterpart in the Soviet shuttle was the RD-0120, similar in specific impulse, thrust, and chamber pressure specification to the SSME, but with some differences that reduced complexity and cost at the expense of increased engine weight.
Full-flow staged combustion cycle
Full-flow staged combustion (FFSC) is a variation on the staged combustion cycle where all of the fuel and all of the oxidizer pass through their respective power turbines. A small amount of fuel and oxidizer is swapped and combusted to supply power for the turbines.
Both turbines run cooler in this design since more mass passes through them, leading to a longer engine life and higher reliability. The design can provide higher chamber pressures and therefore greater efficiency. An intrapropellant turbine seal is also eliminated. Full gasification of components leads to faster chemical reactions in the combustion chamber and, as compared to the partial staged combustion cycle, it results in an increase of specific impulse up to 10–20 seconds (e.g., RD-270 and RD-0244).
Because FFSC engine designs typically limit the nominal characteristic conditions in the engine, engine life is generally longer in FFSC designs. Up to 25 flights are anticipated for one particular engine design studied by the ESA.
Early demonstration tests
Prior to 2014, only two full-flow staged combustion rocket engines have ever progressed sufficiently to be tested on test stands: the Soviet Energomash RD-270 project in the 1960s and the US government-funded Aerojet Rocketdyne Integrated powerhead demonstration project in the mid-2000s.
The Raptor engine currently under development by SpaceX is a full-flow staged combustion engine that will be powered by liquid methane and liquid oxygen This is a distinct departure from the 'open cycle' gas generator system and LOX/kerosene propellants used by the existing set of SpaceX Merlin engines used for orbital launches.
As in other full-flow designs, Raptor will flow 100 percent of the oxidizer (with a low-fuel ratio) to power the oxygen turbine pump, and 100 percent of the methane fuel (with a low-oxygen ratio) to power the methane turbine pump. Unusually however, both streams—oxidizer and fuel—will be completely in the gas phase before they enter the combustion chamber, making the engine the first gas-gas, full-flow staged combustion engine.
Additional characteristics of the gas-gas full-flow design are projected to further increase performance, reliability, or both:
- eliminating the fuel-oxidizer turbine interseal which is traditionally a point of failure in modern chemical rocket engines
- lower pressures are required through the pumping system, increasing life span and further reducing risk of catastrophic failure
- ability to increase the combustion chamber pressure, thereby either increasing overall performance, or "by using cooler gases, providing the same performance as a [standard] staged combustion engine but with much less stress on materials, thus significantly reducing material fatigue or [engine] weight."
Staged combustion engines include the following:
- SSME—Space Shuttle Main Engine
- NK-33 and derivatives, also known as AJ-26
- RD-270 (full flow)
- Integrated powerhead (full-flow demonstration project)
- Raptor LOX/methane engine (full flow)
Staged combustion engines have been used in:
- Space Shuttle
- Atlas III
- Atlas V
- Antares (rocket)
- N1 (rocket)
- Proton (rocket family)
- Zenit (rocket family)
Staged combustion engines are planned for:
- United States General Accounting Office (1994). Aerospace Plane Technologies: R&D in Japan & Australia. DIANE Publishing. p. 145. ISBN 1-56806-059-9.
- George Sutton, "History of Liquid Propellant Rocket Engines", 2006
- Cosmodrome History Channel, interviews with Aerojet and Kuznetsov engineers about the history of staged combustion
- Sippel, Martin; Yamashiro, Ryoma; Cremaschi, Francesco (2012-05-10). "Staged Combustion Cycle Rocket Engine Design Trade-offs for Future Advanced Passenger Transport". Space Propulsion 2012. ST28-5 (DLR-SART). Retrieved 2014-03-19.
- Belluscio, Alejandro G. (2014-03-07). "SpaceX advances drive for Mars rocket via Raptor power". NASAspaceflight.com. Retrieved 2014-03-09.
- Todd, David (2012-11-22). "SpaceX’s Mars rocket to be methane-fuelled". Flightglobal. Retrieved 2012-12-05. "Musk said Lox and methane would be SpaceX’s propellants of choice on a mission to Mars, which has long been his stated goal. SpaceX’s initial work will be to build a Lox/methane rocket for a future upper stage, codenamed Raptor. The design of this engine would be a departure from the “open cycle” gas generator system that the current Merlin 1 engine series uses. Instead, the new rocket engine would use a much more efficient “staged combustion” cycle that many Russian rocket engines use."
- "SpaceX propulsion chief elevates crowd in Santa Barbara". Pacific Business Times. 19 February 2014. Retrieved 22 February 2014.
- Rocket power cycles[dead link]
- Nasa's full flow stages combustion cycle demonstrator
- Design Tool for Liquid Rocket Engine Thermodynamic Analysis